A metal-clad laminate includes one which is manufactured by bonding an insulating base material and a metal layer to each other via an adhesive or an adhesive film. For example, there is proposed a metal-clad laminate of a three-layer structure in which an insulating base material composed of an aromatic polyimide resin film and a metal layer are bonded to each other via an adhesive film (see Patent Document 1).
Adhesives and adhesive films composed mainly an epoxy based or acrylic resin have hitherto been used as the adhesive or adhesive film. However, since such a resin is inferior in heat resistance, the heat resistance of a product after bonding becomes insufficient, whereby restrictions were generated in subsequent processing condition and use condition.
For that reason, adhesives and adhesive films which are excellent in heat resistance are required. For example, there are disclosed a method in which a solution of a polyimide resin or a polyamic acid is coated on an insulating base material, and the solvent removal and optionally, an imidation treatment are then carried out, thereby forming an adhesive layer with thermo-compression bonding properties; a method in which a solution of a polyimide resin or a polyamic acid is coated on a glass sheet, etc., and the solvent removal and optionally, an imidation treatment are then carried out, thereby forming an adhesive film with thermo-compression bonding properties; and a method in which an adherend such as a metal layer, etc. is subjected to thermo-compression bonding on the thus formed adhesive layer or adhesive film (see Patent Documents 2 and 3). The foregoing adhesive layer forming method is roughly classified into a method of using a solution of a polyimide resin and a method of using a polyamic acid solution.
In the method of using a polyamic acid solution, in order to form an adhesive layer or an adhesive film after coating a polyamic acid solution on an insulating base material or a glass sheet, an imidation process at a high temperature exceeding 300° C. must be carried out. The heat resistance of a metal-clad laminate which is formed while omitting the imidation process is conspicuously low. In the method of using a solution of a polyimide resin, after coating, it may be sufficient that the solvent is merely volatilized, and an adhesive layer or an adhesive film can be formed at a low temperature of up to about 200° C. Accordingly, the method of using a solution of a polyimide resin is advantageous in manufacturing a metal-clad laminate with high heat resistance. Most of conventional adhesive layers which are composed of a wholly aromatic polyimide resin were formed using a polyamic acid solution. In order to obtain a solution of a polyimide resin, a solvent-soluble polyimide resin is necessary.
On the other hand, in a metal-clad laminate, there was encountered a problem that when the amount of a residual volatile component of an adhesive layer to be disposed between an insulating base material and a metal layer is high, blanching, blister, foaming, etc. of the adhesive layer is caused during a soldering step reaching a high temperature of 250° C. or higher, thereby noticeably hindering adhesive properties between the insulating base material and the metal layer (see Patent Document 4). Examples of this residual volatile component of the adhesive layer include water and the solvent which have not been removed in imidation and solvent removal steps during the formation of an adhesive layer or an adhesive film; water to be absorbed from the manufacturing environment; water to be absorbed at the time of dipping in an aqueous solution in an etching step; and the like. Of these, water is especially regarded as a problem. In order to solve the foregoing problem, it is desirable to decrease a coefficient of water absorption which is an index of the water content of polyimide.
Also, there is disclosed a heat fusible polyimide resin containing 1,2,4,5-cyclohexanetetracarboxylic acid skeleton in a molecular main chain thereof, which is obtainable from 1,2,4,5-cyclohexanetetracarboxylic dianhydride or a reactive derivative thereof (see Patent Document 6). Example 1 thereof discloses a transparent, yellow polyimide resin film having a glass transition temperature of 304° C., which is obtained by reacting 1,2,4,5-cyclohexanetetracarboxylic dianhydride or a reactive derivative thereof with diaminodiphenylmethane to form an amic acid, which is then coated and subsequently heated for imidation, and this is further heat molded under a pressure. Also, Patent Document 5 discloses that a transparent, less colored polyimide resin film having a glass transition temperature of 300° C. or higher is obtainable from a polyimide resin solution which is obtained by using diaminodiphenyl ether.
The foregoing polyimide resin having a 1,2,4,5-cyclohexanetetracarboxylic acid skeleton is advantageous from the standpoint of film fabrication because a high molecular weight is relatively easily achieved, a flexible film is easily obtainable, and the solubility in a solvent is sufficiently large. Also, it is extremely useful because an adhesive layer which is flexible and which has sufficient thickness and durability can be easily formed upon being coated.
However, a polyimide resin film described in Patent Document 6 is formed through an imidation step at a high temperature similar to the conventional technology, and therefore, the film is colored. Also, the polyimide resin films described in Patent Documents 5 and 6 involved such a drawback that they are high in coefficient of water absorption and inferior in hygroscopic dimensional stability. There is also an example in which a polyimide resin having a 1,2,4,5-cyclohexanetetracarboxylic acid skeleton is applied to a metal-clad laminate (see Patent Document 7). This polyimide resin is solvent-soluble and is used as an adhesive layer of a metal-clad laminate. However, since the polyimide resin itself is thermoplastic, a metal-clad laminate in which only this polyimide resin is used as an adhesive layer had a difficult point in processability at a temperature exceeding a flow initiation temperature of this polyimide resin. Also, a further enhancement regarding its adhesive strength has been desired. Also, Patent Document 8 discloses a manufacturing method of a solvent-soluble polyimide by polycondensation of an aliphatic tetracarboxylic dianhydride, an aliphatic tetracarboxylic acid or a derivative thereof and a diamine compound in a solvent in the presence of a tertiary amine compound. However, since this polyimide is thermoplastic, too, when used in a metal-clad laminate, it causes the same difficult point.    [Patent Document 1] JP-A-55-91895    [Patent Document 2] JP-A-5-32950    [Patent Document 3] JP-A-5-59344    [Patent Document 4] JP-A-2001-329246    [Patent Document 5] JP-A-2003-168800    [Patent Document 6] U.S. Pat. No. 3,639,343    [Patent Document 7] JP-A-2005-1380    [Patent Document 8] JP-A-2005-15629